1. Field of the Invention
The present invention relates to a halogenation process for star-branched polymers with improved neutralization of the hydrogen halide by-product resulting from the halogenation. The term "star-branched" relative to polymers is used herein to refer to polymers comprising a plurality of branches, typically linear, emanating from the same or closely spaced location on the polymer backbone.
2. Description of Information Disclosures
Halogenation processes in which a polymer is reacted with a halogenation agent to produce a halogenated polymer containing chemically bound halogen and a hydrogen halide by-product followed by neutralization of the hydrogen halide by-product by contacting the halogenation reaction product with an alkaline material which reacts with the hydrogen halide by-product are well known.
European Patent Application 88311634.5 filed Dec. 8, 1988 (Publication No. EP 0320 263 A2 published Jun. 14, 1989), the teachings of which are hereby incorporated by reference, discloses star-branched copolymers of a C.sub.4 to C.sub.7 isomonoolefin and a C.sub.4 to C.sub.14 multiolefin (i.e., star-branched butyl rubber), and a process for halogenating these star-branched polymers, including a halogenation process in which the star-branched copolymer is dissolved in an organic solvent.
U.S. Pat. No. 3,099,644 (Parker et al.) is directed to a halogenation process for butyl rubber. It discloses that in the early production of chlorobutyl rubber before the current solvent replacement process was developed, the practice was to recover dry regular butyl rubber and then redissolve it to prepare a solution for halogenation. In recovering regular butyl, the practice is to withdraw a slurry of the polymer in methyl chloride diluent from the reactor and introduce it into a tank of hot water to flash off the methyl chloride and unreacted monomers. This water regularly contains a small amount of polymer dispersing agent such as zinc stearate or calcium stearate or other metal soap to prevent the polymer particles from agglomerating and to keep them dispersed in the water. Much of this dispersing agent is picked up by the polymer so that when that process is used, the butyl solutions submitted to halogenation, unless specially treated, contain small amounts of these polymer dispersing agents. These act as emulsifying agents when the polymer solution is treated with dilute solution of caustic in the neutralization step (col. 3, lines 58 to 72). The presence of these dispersing agents was perceived as a problem, as discussed in Parker et al. Zinc stearate was particularly objectionable because it caused scorching and prevulcanization of the halobutyl during finishing as discussed in Eby (U.S. Pat. No. 2,958,677 col. 2 lines 1 to 5) and its use was eventually abandoned. However, even calcium stearate caused extremely slow settling which was perceived as undesirable (col. 3 line 70 to col. 4 line 2 of U.S. Pat. No. 3,099,644) and necessitated the use of very special and restrictive conditions (i.e. pH control and long residence time settlers, as disclosed on the Parker et al. Patent) to make the process operable.
Subsequently, when the currently used solvent replacement process was developed, the solution for halogenation was prepared directly from the methyl chloride slurry exiting the polymerization reactors by solvent replacement and the polymerization solution contained no "undesirable" stearates. The finding, in accordance with the present invention, that the presence of stearic acid or its in-situ generation in the halogenation solution prior to halogenation actually improves neutralization rate is surprising and contrary to the teachings of prior art.
When the halogenation of the star-branched copolymer of a C.sub.4 to C.sub.7 isomonoolefin and C.sub.4 to C.sub.14 multiolefin, hereinafter referred to as "star-branched butyl rubber", is performed by contacting a solution comprising the star-branched butyl rubber and an organic solvent, the solution also comprises solid particles of unreacted branching agent that was used to prepare the star-branched butyl rubber. Although not wishing to be bound by theory, it is believed that at least a portion of the unreacted solid branching agent particles adsorb a portion of the hydrogen halide by-product of the halogenation process. The solid branching particles with or without adsorbed hydrogen halide are believed to be the cause of the observed decreased rate of neutralization of the hydrogen halide by-product of the halogenation of star-branched butyl rubber because during the neutralization step of the reaction product of the halogenation process of the present invention, two phases exist, namely, a hydrocarbon phase and a water phase. The hydrogen halide by-product diffuses into the water phase to be neutralized by the aqueous alkaline material. However, it is more difficult for the hydrogen halide adsorbed on the solid branching agent particles to diffuse into the aqueous phase during the neutralization process. Hence, the neutralization rate of star-branched butyl rubber is slower than the neutralization rate of conventional butyl rubber (i.e., non-star-branched butyl). A slow neutralization rate has a number of disadvantages, such as a reduced throughput rate and or the necessity of providing additional equipment (more or longer pipes) to permit longer residence time to complete the neutralization.
It would be desirable to increase the rate of neutralization to increase the throughput rate, to require less neutralization residence time, and to minimize the possibility of incomplete neutralization and thereby increase the stability and consistency of the halogenated copolymer.
It has been found that one method that could be used to increase the neutralization rate would be to remove the solid particles of unreacted branching agent prior to the halogenation step from the solution comprising the star-branched butyl rubber, the solid unreacted branching agent particles and the organic solvent. It has also been found that another method that could be used to increase the neutralization rate is to change the nature of the unreacted solid branching agent particles prior to the halogenation step to increase their diffusion from the hydrocarbon phase into the water phase during the neutralization step.
One method of removing the unreacted solid particles of branching agent from the solution comprising the solid particles, the star-branched butyl rubber and an organic solvent comprises adding another solvent (i.e., a cosolvent) to said solution prior to the halogenation step. Suitable solvents to dissolve the branching agent include toluene, methylene chloride, methyl chloride and mixtures thereof in an amount sufficient to dissolve the solid particles present in said solution. Although such a method results in an increased neutralization rate of the hydrogen halide by-product of the halogenation step, it has disadvantages, particularly for large scale production, since it requires the use of two different solvents and subsequent recovery of the two different solvents.
Therefore, a method that would change the nature of the unreacted solid branching agent particles to make them hydrophilic would be preferable to the method of dissolving the solid particles by use of two different solvents.
It has now been found that the neutralization rate of the hydrogen halide by-product of the halogenation of a star-branched butyl rubber can be increased by the addition of water and specified wetting agent or wetting agent precursors prior to the halogenation step, to a solution comprising the star-branched butyl rubber, the solid unreacted branching agent particles and an organic solvent, thereby producing hydrophilic solid branching agent particles in said solution.